1. Field of the Invention
The invention relates to granules based on pyrogenically prepared silicon dioxide, the method for their preparation and the use thereof as catalyst supports and for glass applications.
2. Description of Background Art
The preparation of pyrogenic silicas or silicon dioxides from SiCl4 by means of high-temperature-or flame hydrolysis is known (Ullmanns Enzyklopädie der technischen Chemie, 4th Edition, Volume 21, page 464 (1982)).
Pyrogenic silicon dioxides are distinguished by having extremely fine particles, high specific surface (BET), very high purity, spherical particle shape and the absence of pores. On account of these properties pyrogenically prepared silicon dioxides are attracting increasing interest as supports for catalysts (Dr. Koth et al., Chem. Ing. Techn. 52, 628 (1980)). For this application the pyrogenically prepared silicon dioxide is shaped by mechanical means, for example, tabletting machines.
The shaping of pyrogenically prepared silicon dioxide into sprayed granules also by means of spray drying, to obtain a starting material for sintered ceramic materials, is also known (DE-A 36 11 449).
It is also known that silicon dioxide pyrogenically prepared in an electric arc may be shaped by means of spray drying into sprayed granules, which can be used as adsorption media or else as catalyst supports (DE-A 12 09 108).
The subjection of pyrogenically prepared silicon dioxide to a gel process and the subsequent shaping into granules by means of spray drying is also known. These granules, after coating with chromium oxide, are used in the polymerisation of ethylene (EP-A 0 050 902, U.S. Pat. No. 4,386,016).
Furthermore, the use of precipitated silicon dioxide as a catalyst support for the catalytic polymerisation of olefins is known (WO 91/09881).
The known sprayed granules of pyrogenically prepared silicon dioxides have the disadvantage that they are not optimally suitable for use as catalyst supports, for example, in the production of polyethylene.